The present invention relates to a copolymer suitable for use in the production of a water content soft contact lens having an excellent oxygen permeability.
Various kinds of soft contact lenses have hitherto been known, and their representative examples are, for instance, a water content soft contact lens made of a polymer comprising predominantly 2-hydroxyethyl methacrylate (hereinafter referred to as "2HEMA"), a soft contact lens having a high water content made of a copolymer comprising predominantly N-vinylpyrrolidone (hereinafter referred to as "NVP") and an amido group-containing monomer, a water non-content soft contact lens made of silicone rubber comprising predominantly polydimethylsiloxane, and the like.
Although these soft contanct lenses are superior in wearing sensation and affinity for the eye tissue, especially the corneal tissue, to the conventional water non-content hard contact lenses made of a polymer of methyl methacrylate, they have the following defects.
For example, the water content soft contact lenses made of a polymer of 2HEMA cannot supply enough oxygen required in metabolism of corneal tissue from the atmosphere to the cornea through the lens materials, in other words, the oxygen permeability is bad. Consequently, wearing of such lenses for a long period of time causes metabolic trouble of the corneal tissue.
The soft contanct lenses made of a copolymer of NVP and an amido group-containing monomer having a water content of about 60% to 80% are superior in water content and oxygen permeability to the above soft contact lenses made of a polymer of 2HEMA. However, they have the defects such as (a) the strength of the contact lens materials is lower than that of low to medium-water content contact lens materials, (b) the contact lens is liable to be contaminated due to the accumulation of inorganic and/or organic substances contained in lacrimal fluid and (c) a stable absolute visual power is not obtained due to the difficulty of maintaining a constant lens contour because of evaporation of water upon wearing.
The silicone rubber contact lenses made of polydimethylsiloxane are excellent in oxygen permeability. In the contrary, the surface of them is so water-repellent that it should be modified into hydrophilic generally by corona discharge treatment. The thus treated lens surface, however, is liable to adhere contaminants such as proteins contained in lacrimal fluid. Consequently, these contact lenses make a trouble in the corneal tissue due to a strong sensation of foreign bodies, especially a burning sensation.
Furthermore, soft contanct lenses made of a copolymer of a siloxanylalkyl ester of acrylic acid or methacrylic acid and a hydrophilic monomer have been reported. The above siloxanylalkyl ester, however, is less in compatibility with a hydrophilic monomer due to its essentially strong hydrophobic property. Therefore, a copolymer sufficiently satisfied for contact lens materials has never been obtained.
Japanese Unexamined Patent Application No. 40324/1981 discloses a soft contact lens made of a copolymer of a specific siloxanylalkyl ester having hydroxyl group in its molecule and a hydrophilic monomer. Even by employing the above prior art, however, the essential hydrophobic property of the above siloxanylalkyl ester is not solved, and the thus obtained copolymer shows low strength. Therefore, it is difficult to make a more excellent soft contact lens being practically available.
It is an object of the present invention to provide a hydrophilic copolymer suitable for a soft contact lens having a high oxygen permeability and an excellent affinity for the eye tissue.
A further object of the present invention is to provide a copolymer suitable for an oxygen permeable soft contact lens which can be worn continuously for a long period of time.
These and other objects of the present invention will become apparent from the description hereinafter.